Vehicle instrument clusters are normally produced as boxed units having a front panel in which is provided a plurality of apertures for use as display areas, which are associated with the instruments of the cluster. The cluster, in addition to incorporating a plurality of instruments, typically contains one or more printed circuit boards supporting the appropriate electronic circuitry to drive the cluster, a plurality of housings containing the instruments, a plurality of light bulbs for lighting the instrument display areas, and cables and strengthening pillars and the like.
Each display area is normally covered by a coverlay made of transparent or translucent polycarbonate material, which supports a set of graphic symbols pertinent to the particular instrument with which the coverlay is associated. The graphic symbols are printed on the front surface of the coverlay and are formed of a translucent material, whilst the areas of the coverlay not covered by graphic symbols are masked off with an opaque material, for example a black ink.
The graphic symbols are illuminated by the light from the light bulbs inside the instrument cluster shining through the translucent material of the graphic symbols.
A problem encountered with such types of instrument cluster is that the components inside the cluster tend to affect the light reaching the display area, for example by reflecting the light to give bright areas or by obscuring the light to create dark areas. As a consequence, the display area is not evenly lit, resulting in the graphic symbols on the coverlay being illuminated to different extents. It is therefore usually necessary to compensate for the differences in light levels of the graphic symbols by providing a light mask behind the coverlay to dim the over-lit symbols, thereby to balance the intensity levels of all the symbols, and to ensure that the display is of satisfactory quality.
Compensation for such differences in the light levels across each display area is generally carried out subjectively, by determining which graphic symbols are lit more than others, and what level of masking is required to balance the lighting levels. Masking paint, of semi-opaque material, is then applied to the coverlay, behind the appropriate over-lit symbols.
The masking may consist of one or more masking portions having a size and shape suitable for masking the particular graphic symbol with which they are to be associated.
Such compensation is both difficult and time consuming and does not tend to lead to satisfactory results.